Various suspension systems have been used to lower a trailer or agricultural structure at or near the ground, utilizing hydraulic lifters. However, these systems have complicated lifting mechanisms with more movable parts, such as rotatable rocker arms, which are subject to uneven stresses in use.
Among the United States patents are the following:
______________________________________ 5,161,814 of Walker 3,697,098 of Fisher 5,137,297 of Walker 3,536,339 of Fichtenberg 4,900,055 of Wright 2,771,303 of Frazier 4,886,290 of Pourchon 2,297,992 of Swim 4,326,594 of Oka ______________________________________
U.S. Pat. Nos. 5,137,297 of Walker and 5,161,814, also of Walker, disclose trailerable structures with retractable towing hardware, including complicated rocker arms.
U.S. Pat. No. 4,900,055 of Wright discloses a hydraulic retractable and extensible wheel suspension, with through axles, which has rocker arms, has complicated levers and links, and which can't be lowered all the way to the ground.
U.S. Pat. Nos. 4,326,594 of Oka and 3,536,339 of Fichtenberg disclose raising and lowering mechanisms for farm implements with complicated rotatable rocker arms.
U.S. Pat. No. 2,297,992 of Swim discloses the vertical adjustment of a frame with complicated scissors jacks, and which can't be completely lowered to the ground.
U.S. Pat. No. 2,771,303 of Frazier describes a conventional wheel suspension system with leaf springs, which does not lower to the ground, and U.S. Pat. No. 3,697,098 of Fisher discloses a coupling device for a mobile home which does not lower to the ground.
U.S. Pat. No. 4,886,290 of Pourchon describes a complicated device with hydraulic lifers encompassing rotatable links and rocker arms.
British Patent GB2204537A of Ratcliffe discloses a temporary trailer having a box-like body wherein one end of the tow bar leaf spring is removed down adjacent to the ground by removing a cotter pin to lower the leaf spring to the ground. The rotation of the leaf spring presents uneven mechanical stress when lowering the trailer to the ground.
French Patent 2560-489-A of Lerin discloses a livestock transporter which is removable to the ground. However, it appears to be removable by rocking arm 12, which is similar to the rocking arm mechanisms of the above noted U.S. patents.
British Patent, GB21949025A of Tomecek, discloses a lowerable trailer suspension system which, renders the wheels inoperative, so that they can be lowered to the ground with rotatable rocker arms.
Japanese Patent 63-232082(A) of Kawanami discloses a trailer system with complicated X-shaped links 3 which can be compressed for lowering the trailer to the ground.